Monte Carlo calculation of rectal dose when using an intrarectal balloon during prostate radiation therapy

Air-filled intrarectal balloons can be used to localize and immobilize the prostate for radiation therapy, allowing dose escalation to the prostate and reducing the probability of radiation proctitis, but also introducing potentially significant heterogeneity. We compare the Eclipse treatment planning system (TPS) with Monte Carlo (MC) simulations for 5 patients to assess how well a conventional TPS includes the effect of the balloon on doses near the rectum. The MC results show that, for a 27-Gy prescription to the 95% isodose line, Eclipse overestimates the volume of the rectum receiving more than 26 Gy (96%) by 2 approximately 10 cc and the volume of the rectum receiving between 12 approximately 15 Gy by 10 approximately 20 cc. Differential dose volume histograms are also computed and compared for individual fields in the anterior expansion of the rectum, and the TPS is again shown to predict higher mean dose in the region by 0.3 approximately 1.0 Gy.